It is often desirable to scan a test subject and/or a three-dimensional area around a test subject using some form of imaging technique. Acoustical holography, for example, is a method often used to characterize the surface velocities and acoustic pressures of coherently vibrating structures such as engines and gearboxes.
For aeroacoustic noise sources such as jets with multiple partially-correlated source mechanisms, scan-based techniques using reference and response transducers and singular value decomposition have been applied to acoustical holography to decompose a noise source into partial fields. The partial fields can reconstruct an overall sound field and also provide a near-field representation of the source that can help in understanding the physics of jet noise.
The acoustic source characteristics of jet plumes from high performance military fighter engines are not well defined, however. This is due to the difficulty in making a complete set of descriptive acoustic measurements characterizing the size, intensity, directivity, and distribution of the acoustic source (i.e., jet plume).
Acoustic near-field acoustic holography concepts have been proposed for full-scale jet engines. An acoustic hologram is a phase-locked “picture” of a spatially coherent pressure (or velocity) field that corresponds to an equivalently vibrating surface at the measured points. Acoustic holograms are typically presented on a frequency by frequency basis. By making successive array measurements (“scans”) over a sufficiently large hologram surface in a source-free region, this technique allows, in theory, for an inverse propagation of the wavenumber spectrum of the measured surface pressures to any surface closer to (but still containing) the source, as well as a complete description of the sound field further away from the source. Aeroacoustic sources such as jets do not actually produce a spatially coherent pressure field, so an acoustical holography system for high-speed jets must approximate the sound source as a number of mutually incoherent acoustic holograms (“partial fields”).
Acoustic near-field acoustic holography concepts have yet to be realized for full-scale jet engines, in part because of the large number of sensors required. For example, a 3-D microphone measurement array to fully characterize the acoustic field around an aircraft and jet plume might require hundreds or even thousands of individual acoustic sensors.
It is therefore desirable to provide imaging systems and methods that are efficient, fast, and allow three-dimensional scanning to be performed using a reduced number of sensors. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.